The present invention relates to navigating using a global positioning system (GPS), and in particular to systems for providing assisted-GPS and for providing GPS time recovery, especially in weak signal conditions.
The operation of a global positioning system (GPS) receiver is predicated on the receiver having a precise value for GPS time; without such a precise value, the ranges (from the receiver to one or more of the GPS satellites) computed by the receiver are inaccurate, since they are determined simply as the difference between the time of transmission and time of receipt multiplied by the speed of light. GPS navigation therefore relies on all elements of GPS (including both satellites and GPS earthbound receivers) having a clock synchronized to GPS system time, which is an approximate version of so-called coordinated universal time (UTC). GPS (more specifically, the GPS ground monitoring network) disseminates corrections to each satellite (which uses a high accuracy local clock) to account for the bias and offset of the satellite clock compared to GPS system time, and the satellites provide these corrections in the navigation message they each broadcast; therefore all satellites are synchronized. A GPS receiver, on the other hand, determines the offset of its local clock as part of the solution of the GPS receiver position. In doing so, however, to the extent that the GPS local clock is significantly out of synchronization, the calculation of the GPS receiver position is prolonged. Therefore, it is advantageous, in general, to provide a means of synchronizing a GPS receiver clock to GPS system time.
In sufficiently weak GPS signal conditions, a GPS receiver cannot determine GPS time unassisted. In such conditions, either the exact GPS time has to be recovered to carry out positioning, or positioning must stop. To avoid having to halt positioning, time recovery must be assisted, and there are many ways to assist a GPS receiver in carrying out time recovery, one being to deliver the exact GPS time from a cellular network, such as a GSM network.
Unfortunately, a standard GSM network (and also a third generation wideband code division multiple access network) is not synchronized to any universal time reference, since cellular communication is not per se a navigation tool and therefore there is no need for a universal synchronized time in providing cellular communications. The only time synchronization that is often needed is time slot synchronization, where a mobile station synchronizes itself to a particular base station schedule in order to keep its own transmission in its assigned time slot and to pick up the messages from the base station intended for the mobile station. Such synchronization is therefore (and need only be) relative (between a mobile station and a base station) as opposed to universal. To enable deriving a universal time from such a cellular network, new equipment and new messages are needed.
There is a device that provides a GPS/GSM timing relationship (mapping); it is called a Location Measurement Unit (LMU); an LMU can be thought of as a specialized GPS receiver located at a cellular base station, a GPS receiver adapted to time-stamp with GPS system time communication signal bursts to mobile stations. An LMU time-stamps with GPS time the communication signal bursts from base stations. An LMU provides to a GPS receiver (a GPS receiver configured to make use of the LMU provided information) the help needed in weak signal conditions in constructing GPS time (i.e. in synchronizing with GPS system time). The LMU provides a so-called reference time information element, in which it indicates which GSM signal frame, time slot and bit are to be used as a time reference point, according to which for example a mobile station can remove the GSM system delay, and so recover exact GPS time.
For typical GPS receiver positioning accuracy, GPS time must be known to within xcx9c10 xcexcs in the receiver. Such accuracy is difficult to achieve using an LMU-based system for time synchronization, but co-owned U.S. application Ser. No. 09/777,521, filed Feb. 5, 2001, hereby incorporated by reference, provides a solution. According to the art prior to that application, a GPS receiver is adapted to make use of LMU assistance by including a cellular component, in addition to the GPS component, that responds to the LMU message (including picking up the actual trigger in the indicated signal frame, time slot, and bit) (see FIG. 1). The two components communicate via a software messaging layer. There are, however, significant random delays in communicating messages over a software messaging layer, i.e. internal delays arising because of use of the software messaging layer for various other tasks besides only providing LMU time synchronization information. (Random delays occur inside buses used by a GPS receiver, buses where messages are transmitted from one software module to another using a dedicated software messaging architecture. Such delays can be tens of milliseconds in duration.) U.S. application Ser. No. 09/777,521 provides a special hardware connection (see FIG. 1) between the GPS component and the cellular component that is used to signal to the GPS component the precise time of arrival of a time-stamped frame indicating an instant of time having a value according to GPS time that is conveyed by another frame to the cellular component and then communicated over the software messaging layer to the GPS component.
Thus, an LMU is used, especially in poor signal conditions, to synchronize a GPS component to GPS time. In many places, however, base stations are not equipped with an LMU. In such situations, it would be advantageous if a GPS receiver, including both a GPS component and a cellular component, that could synchronize itself to GPS time because of operating in favorable signal conditions, could then communicate GPS time to GPS receivers (also including both a cellular component and a GPS component) operating in less favorable signal conditions and in places where services from an LMU are not available.
Accordingly, in a first aspect of the invention, an apparatus is provided comprising: a cellular module, responsive to a portion of a downlink frame signal conveying a cellular frame provided by a cellular communication system; and a main module, responsive to ranging signals from beacons of a positioning system in which time is kept according to a predetermined time reference; the apparatus characterized in that: the cellular module provides a trigger signal indicating the arrival of the portion of the downlink frame signal, and is further responsive to a signal indicating information for relating the instant at which the trigger signal is provided to a time value according to the predetermined time reference; and the main module provides the signal indicating the information for relating the instant at which the trigger signal is provided by the cellular module to a time value according to the predetermined time reference; wherein the trigger signal is provided over a connection between the cellular module and the main module so constructed that any delay between sending and receiving the trigger signal is either negligible or can be determined.
According to the first aspect of the invention, the apparatus may be further characterized by the main module comprising a timing register and a local clock, the timing register responding to the trigger signal by reading the local clock to determine a local trigger time and holding the local trigger time until being reset or until another trigger signal is issued by the cellular module. The apparatus may be further characterized in that the main module further comprises a main processor, responsive to the ranging signals and to the value stored in the timing register and to the local time as indicated by the local clock, for determining a navigation solution including a preliminary time value according to the predetermined time reference, the preliminary time value being a time value for the instant of time for which the navigation solution is determined, and for adjusting the preliminary time value by the difference in time according to the local clock when the timing register was triggered and when the navigation solution was determined, thereby providing the time value, according to the predetermined time reference, for the instant when the timing register was triggered.
Also according to the first aspect of the invention, the apparatus may be further characterized in that it further comprises a time slot counter, and the trigger pulse is provided when the time slot counter indicates a new time slot.
Also according to the first aspect of the invention, the apparatus may be further characterized in that it further comprises a data bit counter, and the trigger pulse is provided when the data bit counter indicates a new data bit.
Also according to the first aspect of the invention, the apparatus may be further characterized in that the main module is a global positioning system (GPS) module providing functionality for operation as a GPS receiver.
Also according to the first aspect of the invention, the apparatus may be further characterized in that the cellular module provides to a serving base station the information for relating to a time value according to a predetermined time reference, the instant at which the trigger signal is provided. Further, the main module may also provide a position for the instant when the trigger signal was received by the main module, and the cellular module may be responsive to the position, and provide the position to the serving base station.
Also according to the first aspect of the invention, a system is provided comprising an apparatus according to the first aspect of the invention, which receives a cellular frame, and further comprising a cellular base station, for providing the cellular frame.
Also according to the first aspect of the invention, a system is provided comprising an apparatus according to the first aspect of the invention, which receives ranging signals from beacons of a positioning system, and further comprising the beacons of the positioning system that provide the ranging signals.
In a second aspect of the invention, a method is provided for having a ranging receiver, able to be used with a positioning system and including a cellular module and a main module, provide information to a serving base station of use in enabling the serving base station to synchronize to time according to the positioning system, the method characterized by: a step in which the cellular module detects a portion of a time-stamped frame and applies a trigger signal to a connection linking the cellular module to the main module, a connection such that any delay between applying a signal and receiving the signal is either negligible or can be determined; and a step in which the main module sends to the cellular module a time value according to the positioning system for the instant when the main module received the trigger signal, based on performing a navigation solution yielding a time value for an instant of time able to be related to the instant of time when the trigger signal was received using a local clock accessible to the main module.
According to the second aspect of the invention, the method may be further characterized by a step in which the main module determines the difference in time between the time for which the navigation solution was obtained and the time when the trigger signal arrived at the main module, based on using a local clock and a timing register latchable by the trigger signal so as to hold a time value according to the local clock for when a trigger signal arrives.
Also according to the second aspect of the invention, the method may be further characterized in that in the step in which the main module sends to the cellular module a time value, the main module also sends the cellular module a signal indicating information about the position of the ranging receiver.
The invention allows a time stamp to be made in a mobile device, so that the mobile device serves the function of an LMU, thus eliminating the need of having a separate LMU. In addition, the accuracy of the invention is improved over what is typically provided by an LMU; the accuracy provided by the invention is for example in the range of 1-2 xcexcs.